1. Field of the Invention
The present invention relates to a liquid crystal composition and a display device using the same. More particularly, the present invention relates to a novel liquid crystal composition for which the electric field can be improved, and a liquid crystal display device which uses the same. The present invention also relates to a liquid crystal device for use in a liquid crystal-optical shutter or the like and a display device which uses the above-described liquid crystal device as the display device.
2. Description of the Prior Art
Hitherto, liquid crystal has been used in a variety of fields as an electro-optical device. A major portion of the liquid crystal devices, which have been put into practical use, uses the TN (Twisted Nematic) type of liquid crystal described in "Voltage Dependent Optical Activity of a Twisted Nematic Liquid Crystal" in Applied Physics Letters, Vol. 18, No. 4 (Feb. 15, 1971), pp 127 to 128 written by M. Schadt and W. Helfrich.
Each of the above-described liquid crystal devices utilizes the dielectric array effect of the liquid crystal with which the direction of the average molecular axis is oriented into a specific direction by an applied electric field because of the dielectric anisotropy of the liquid crystal molecule. However, since its optical response speed is limited to an unsatisfactory speed of a unit of milliseconds, it cannot be widely used in a multiplicities of fields. On the other hand, in consideration of the cost reduction and the manufacturing yield, it is most preferable that a structure driven by a simple matrix system be employed when the liquid crystal is applied to a large-size flat display. The simple matrix system employs a configuration of the electrodes arranged in such a manner that the scanning electrode groups and the signal electrode groups are arranged so as to form a matrix. Furthermore, the matrix thus arranged is driven by a time division drive system arranged in such a manner that address signals are successively and periodically applied to the scanning electrode groups, and a predetermined information signal, in synchronization with and in parallel to the address signals, is applied to the signal electrode groups.
However, a structure arranged in such a manner such that the above-described TN-type liquid crystal is employed as the devices for the above-described drive system encounters a problem in that the electric field is definitely applied to a region where the scanning electrode is selected but no signal electrode is selected or a region where the scanning electrode is not selected but the signal electrode is selected (a so-called "half selection point").
If the difference between the voltage applied to the selected point and the voltage applied to the half selection point is sufficiently large and the voltage threshold necessary to cause the liquid crystal molecules to be vertically disposed is determined to be the intermediate voltage of the above-described two voltage levels, the display devices can work normally. However, a structure arranged to have an enlarged number (N) of the scanning lines encounters a problem in that the duty ratio, which is the time in which an effective electric field is applied during the scanning of the entire surface of the frame (1 frame), will be decreased at a rate of 1/N.
Therefore, the voltage difference, which is an effective value, between the selected point and the non-selected point can be reduced inversely in relation to the number of the scanning lines. As a result, unsolved problems remain in that the image contrast can be lowered and the crosstalk is generated undesirably.
The above-described problems are inevitable problems taken place in a structure arranged in such a manner that liquid crystal having no bistability is driven by utilizing a time accumulation effect (that is, scanning is performed repeatedly), where the stable state is defined to be a state in which liquid crystal molecules are horizontally arranged with respect to the surface of the electrode and they are vertically oriented for only the time in which the electric field is effectively applied thereto.
In order to overcome the above-described problem, there have been disclosed a variety of methods such as a voltage averaging method, a dual frequency drive method and a multi-matrix method and the like. However, the enlargement of the frame of the display device and the raising of the density have not been satisfactorily realized because of the limitation of the number of the scanning lines.
In order to overcome the above-described problems experienced with the conventional liquid crystal device, a disclosure has been made by Clark and Lagerwall in Japanese Patent Laid-Open No. 56-107216 and U.S. Pat. No. 4,367,924 on a structure arranged in such a manner that a liquid crystal device has bistability.
As the bistable liquid crystal, a ferroelectric liquid crystal having a phase chiral smectic C (phase SmC*) or phase H (phase SmH*) is usually employed.
The ferroelectric liquid crystal has a bistability composed of first and second optical stable states with respect to the electric field. Therefore, it exhibits characteristics, which are different from the above-described TN type liquid crystal, such that the liquid crystal is oriented in the first optical stable state with respect to an electric field vector and oriented in the second optical stable state with respect to another electric field vector. Furthermore, the liquid crystal of the type described above selects either of the above-described two stable states in response to the applied electric field. In addition, it has the characteristic (the bistability) that the present state is maintained if there is no electric field applied.
The ferroelectric liquid crystal further exhibits high response performance. The reason is that the spontaneous polarization of the ferroelectric liquid crystal and the applied electric field directly interact with each other, causing the transference of the status of the orientation to be induced. The thus realized response speed is faster than the response speed realized by the interaction between the dielectric anisotropy and the electric field by three or four orders.
As described above, since the ferroelectric liquid crystal originally exhibits these excellent characteristics, the above-described problems experienced with the conventional TN type device can be overcome to a certain degree. In particular, it can be expected to be applied to a high speed optical shutter or a high density and/or large frame display. Therefore, liquid crystal materials having the ferroelectric characteristics have been widely researched and developed. However, there has not been developed a liquid crystal material having satisfactory characteristics such as low temperature operationability and high speed response performance so as to be used in a liquid crystal device.
The relationship among the response time t, the level of the spontaneous polarization Ps and the viscosity h is governed by the following equation II. ##EQU1## wherein E is the applied electric field.
Therefore, the response speed can be raised by any of the following methods:
(a) Raise the spontaneous polarization level Ps. PA1 (b) Reduce the viscosity .eta.. PA1 (c) Enlarge the applied electric field E.
However, the applied electric field has an upper limit because it is driven by ICs or the like. Therefore, it must be lowered as much as possible. As a result, the viscosity .eta. must be lowered or the spontaneous polarization level Ps must be raised.
In general, in a ferroelectric chiral smectic liquid crystal compound having a large spontaneous polarization, the internal electric field generated by the spontaneous polarization is large, causing a multiplicity of limitations to be present when the device having the bistable state is structured. In addition, if the spontaneous polarization is enlarged excessively, the viscosity can also be enlarged. Therefore, the response speed cannot satisfactorily be raised.
The TN liquid crystal material is oriented in a nematic phase state by using an orienting film which has been subjected to a simple rubbing orienting process.
On the other hand, when the SmC* liquid crystal material is subject to such a rubbing orientation process, zigzag orientation occurs and the orientation becomes defective in the vicinity of the gap spacer (spacer beads or the like) in the liquid crystal cell.
Furthermore, the defective orientation will easily be generated due to the difference in the rubbing status of the orienting film caused by the projections and pits in the surface of the oriented film which are formed due to the components of the liquid crystal cell.
The inventors of the present invention considers the reason for the above-described fact lies in that the SmC* phase is the phase which has passed a plurality of phase transitions starting from the isotropic phase, and that the SmC* phase more approximates a crystal phase in comparison to the nematic phase.
The above-described defective orientation deteriorates the bistability exhibited by the SmC* liquid crystal material and causes the contrast of the image quality to be lowered and the crosstalk to be enlarged.
As described above, in order to put the ferroelectric liquid crystal device into practical use, there has been a desire for a ferroelectric chiral smectic liquid crystal component which has low viscosity and high speed response and which can easily be oriented by a simple rubbing orienting process and which exhibits uniform monodomain orientation without defects.